A two phase gas/liquid or liquid/liquid mixture is one in which the gas and liquid or the liquid components retain their original physical characteristics even though mixed together. Bubbles of an undissolved gas distributed in a liquid or insoluble globules or droplets of a liquid dispersed in a liquid generally exemplify the two phase mixtures contemplated here.
In certain industrial operations, it is desirable to supply an essentially uniform mixture at components to more than one location, i.e., to maintain the initial ratio of gas to liquid or liquid to liquid from the beginning of the operation through to the use point. The in-situ leaching of uranium, copper, nickel, or other minerals from their ores is one such operation.
In-situ leaching is a well known technique for recovering metal values under ground. This process involves injecting a lixiviant or leach solution into one or more wells where the lixiviant is forced into adjoining ore zones containing the desired mineral. A typical in-situ leaching operation may involve twenty to several hundred wells. The mineral dissolves in the lixiviant and the mineral bearing or pregnant lixiviant is then pumped to the surface via the same or other wells drilled in the strata into which the barren lixiviant is injected. This dissolved mineral is stripped from the pregnant lixiviant by ion exchange or other conventional techniques and the barren lixiviant, after appropriate adjustment of its composition, is reinjected into the ore zone.
For certain minerals such as uranium it is necessary to oxidize the ores underground with oxygen or some other oxidant in order to convert the mineral to a state in which it is soluble in the lixiviant. Various means for providing oxygen in either the dissolved or undissolved form are known in in-situ mining. Most in-situ uranium mining operations use some form of down-hole sparging for injection of the oxygen gas into the lixiviant. The oxygen is typically distributed at about 100 to about 300 pounds per square inch gauge (psig) from one or more central liquid storage and evaporation units through a piping system to each of the multitude of injection wells in a given leach field. The oxygen is fed through a flow control valve and flow-metering device located at each injection well-head down through a tube within the well to a sparger or other gas distribution unit at or near the bottom of the well. Lixiviant is fed in from the well-head and flows down the well bore counter-current to the gas bubbles rising up the well. Under the combined pressure of the hydrostatic and dynamic heads, oxygen gas becomes dissolved in the lixiviant in the course of this counter-current flow. The dissolved oxygen is then carried by the lixiviant into the ore zone where the desired oxidation takes place. The common practice for this type of operation is to have parallel arrays for the lixiviant and the oxygen gas such that the lixiviant and the oxygen are fed proportionately through meters and flow control equipment to provide the desired lixiviant/oxygen ratio.
While the use of meters and flow control equipment adds to the cost of the in-situ leaching operation, merely distributing the oxygen and lixiviant throughout a network of injection wells without controls results in an unsatisfactory distribution of both with some wells receiving too little oxygen and others more than can be efficiently utilized.
One technique for avoiding a complex gas/lixiviant distribution system with its multiplicity of meters and control valves and still maintain control is to dissolve the oxygen in the lixiviant at the surface thus providing a single phase fluid which can be distributed throughout the network of pipes and wells without concern for changes in the ratio of gas to lixiviant. This approach, however, is only applicable when the quantity of gas needed for the in-situ leaching of the mineral is such that the gas can be completely dissolved in the lixiviant and maintained in solution, economically, at the pressure and temperature prevailing throughout the system, i.e., at the surface dissolver and in the lixiviant distribution network. In many cases, the required concentrations of gas are such that uneconomically high pressures would be needed to maintain the gas in solution from the dissolver to the point of injection into the ore zone.